1. Field of the Invention
The present invention relates to an aberration correction liquid crystal device and an optical pickup apparatus.
2. Description of the Related Art
Optical discs such as a CD and a DVD from and onto which data is read and written using a laser beam become widely available in general. The optical disc is formed with small indentations called pits on a disc-like substrate for recording digital bit information. The pits are coated with a reflection layer of metal and a transparent, permeable protection layer, the reflection and protection layers being deposited on each other.
A new standard called Blu-ray Disc (BRD) higher in recording density than a DVD is also proposed. In the BRD, a violet laser having a wavelength of 405 nanometers is used to write a signal onto a record track of about a half of that of the DVD (0.32 microns). The BRD makes it possible to write 27-GB data onto a 12-cm single-layer disc and attention is focused on the BRD as the next-generation digital high-definition recording standard.
To read information from an optical disc, an optical pickup apparatus is used. In the optical pickup apparatus, a laser beam emitted from a laser light source is converged on the optical disc through an object lens and reflected light on the optical disc is received and is converted into an electric signal and information written into each pit is read.
In the optical disc pickup, generally a plurality of aberrations occur. There are disclosed relating arts to correct two different aberrations, in JP-A-11-110802 (See pages 5-13; and FIGS. 1-14) and in JP-A-10-269611 (See pages 3-5, FIGS. 1-5).
The document JP-A-11-110802 discloses aberration correction means for correcting comatic aberration occurring due to a tilt of an optical disc, particularly comatic aberrations about two directions of radial direction and tangential direction. The aberration correction means described in JP-A-11-110802 has segment electrodes and common electrodes formed with electrode patterns for correcting aberrations in the different directions.
In JP-A-11-110802, comatic aberrations in the radial direction and the tangential direction always simultaneously occur at the disc recording/playing back time and thus it is necessary to apply voltage to both electrodes for correcting the comatic aberrations in the radial direction and the tangential direction at the same time.
In correcting the aberration, both plus and minus directions exist as the correction direction. For example, to correct tilt of a disc, voltages are applied to electrodes on both sides of the reference electrode depending on the disc tilt direction. That is, the potential of each electrode needs to be made higher or lower than the potential of the reference electrode depending on the disc tilt direction. Taking the electrode configuration shown in FIG. 4 as an example, 3 V and 1 V may be applied to electrode 2 and electrode 3 respectively or 1 V and 3 V may be applied to electrode 2 and electrode 3 respectively with electrode 1 as the center (for example, fixed to 2 V) depending on the disc tilt direction; it is necessary to turn the voltages applied the electrodes 2 and 3 depending on the disc tilt direction. To do this, a circuit for applying voltages in response to the two cases mentioned above becomes necessary.
The document JP-A-10-269611 discloses wave front aberration correction means for correcting wave front aberration (spherical aberration) occurring depending on the thickness (distance) difference between the layers of a multilayer disc. The aberration correction means described in patent document 2 has segment electrodes and common electrodes formed with electrode patterns for correcting aberrations in different directions.
To correct the spherical aberration occurring due to the disc thickness difference, the disc thickness may be larger or smaller than the reference thickness value and to correct the aberration, the remaining electrode potentials relative to the reference electrode potential need to be turned in response to the thickness variation direction. For example, in the electrode configuration in FIG. 4, reference voltage 2 V and 4 V and 1 V may be applied to electrode 2 and electrode 3 and electrode 1 respectively or reference voltage 2 V and 1 V and 4 V may be applied to electrode 2 and electrode 3 and electrode 1 respectively; it is necessary to make the voltages applied to the electrodes 3 and 1 need to be made higher and lower than the voltage applied to the electrode 2 depending on which direction the disc thickness varies from the reference value in. To do this, a circuit for applying voltages in response to the two cases mentioned above becomes necessary.
As described above, according to the related arts disclosed in JP-A-11-110802 and JP-A-10-269611, the segment and common electrodes are provided with the different patterns of electrodes, thereby simultaneously correcting different aberrations occurring at the optical disc recording/playing back time. Thus, voltages must be applied to both segment and common electrodes for correcting both aberrations at the same time.
Since the aberration caused to occur in a liquid crystal device (phase difference) is determined by the voltage difference between segment and common electrodes, it is desirable that one electrode potential should be set to 0 V to provide a larger phase difference. However, to drive both electrodes at the same time, it is virtually impossible to set one electrode to 0 V.
Both plus and minus directions exist as the aberration correction direction and thus any other electrode potential than the reference electrode potential needs to be made higher or lower than the reference electrode potential.